There's no gender socialization in monkeys, right? Then why are the boy monkeys (vervets and rhesus) preferring "boy" toys to "girl" toys? The two sets of experiments have been reported by Psychology Today:

In 2002, Gerianne M. Alexander of Texas A&M University and Melissa Hines of City University in London stunned the scientific world by showing that vervet monkeys showed the same sex-typical toy preferences as humans. In an incredibly ingenious study, published in Evolution and Human Behavior, Alexander and Hines gave two stereotypically masculine toys (a ball and a police car), two stereotypically feminine toys (a soft doll and a cooking pot), and two neutral toys (a picture book and a stuffed dog) to 44 male and 44 female vervet monkeys. They then assessed the monkeys’ preference for each toy by measuring how much time they spent with each. Their data demonstrated that male vervet monkeys showed significantly greater interest in the masculine toys, and the female vervet monkeys showed significantly greater interest in the feminine toys. The two sexes did not differ in their preference for the neutral toys.

I wish they had reported the actual results in this short article. They did report that the boy rhesus monkey preference for "boy" toys was "strong and significant." See also, this related post: Boys' Toys